Spectacular zero-gravity landslide captured in progress on Comet 67P

Comets, it turns out, are rather more active places than we thought. Comet 67P, where we left Rosetta and Philae not too long ago, is one such happenin’ spot. Scientists poring over photos by Rosetta and Philae have unearthed evidence of a spectacular landslide on 67P — and we caught it in action. This is the first time anyone has ever caught direct evidence of a landslide on a comet.

The landslide, which took place in July of 2015, would not have looked like any landslide on Earth when it was happening. Comet 67P is so small it has barely any gravity to speak of. In that environment, instead of tumbling down like an avalanche, much of the material that broke off from the crumbling cliff face created an “outburst.” Some 22,000 cubic meters of material, enough to fill nine Olympic-size swimming pools, floated up above the comet’s surface to form a diffuse cloud of dust and gas.

“These images are showing that comets are some of the most geologically active things in the solar system,” Pajola said. “We see fractures increasing, dust covering areas that were not dusted before, boulders rolling, cliffs collapsing” — all on an object barely wider than the Washington Mall is long.

Here’s the before and after:

What makes comets so active is their lack of atmosphere. With nothing to shield them from the Sun’s direct rays, sometimes the sunlight plays off the landscape in surprising ways. It was winter on 67P’s northern hemisphere, and winter there is about -160°C. But with 67P positioned how it was, the angle of the sunlight as it crested a nearby ridge was such that a focused beam hit the cliffside. Its laser-like intensity raised the local temperature, just there, to 50°C, which was enough of a thermal differential to stress the cliffside until its weakest points just gave out.

Because the patch exposed by the landslide had such a high albedo — that is, it reflected most of the light that struck it — scientists believe the landslide must have revealed water ice from below 67P’s surface. After it was exposed, the ice sublimated from a solid straight into a gas, and the bright spot faded away. Looking at Comet 67P today, the only clue that there ever was such a landslide is a little pile of rubble at the bottom of the cliff.

Philae was a busy little spacecraft, even though it spent its whole tenure on 67P wedged in a crevice. As soon as it landed, scientists started snapping every picture they could get with the lander’s cameras, but they only spied it down in its hidey-hole about a month before the end of the mission.

Among the things Philae was sent to explore were the geological dynamics and composition of rubber-duck-shaped 67P. Scientists have determined that it’s likely that comets like 67P seeded Earth with organic materials relevant to the formation of life on our planet. Water, though, is another story. It’s unlikely that Earth’s water came from comets like 67P.